Burner control systems are designed both to monitor the existence of flame in the supervised combustion chamber and to time and verify the sequence of operations of burner controls and safety interlocks. The safety of the burner operation is a prime consideration in the design of burner control systems. For example, if fuel is introduced into the combustion chamber and ignition does not take place within a reasonable time, an explosive concentration of fuel may accumulate. A burner control system should reliably monitor the existence of flame in the combustion chamber, accurately time a trial-for-ignition interval, inhibit ignition if a false flame signal is present, and shut down the burner in a safe condition whenever a potentially dangerous condition exists. Examples of such burner control systems are shown in U.S. Pat. No. 3,840,322 and U.S. application Ser. No. 769,307, filed on Feb. 16, 1977 by Philip J. Cade, now U.S. Pat. No. 4,137,035.
In burner control systems, different sensors are employed which provide electrical signals to the control system which indicate the presence or absence of various different conditions in the burner. Such sensors may malfunction and result in a dangerous condition occurring in the burner. Thus, a burner control system should verify the proper operation of such sensors. It also occasionally happens that a correctly operating burner is shut down by a burner control system due to a malfunctioning sensor or safety interlock. Upon investigation and discovery of the malfunctioning sensor or interlock, the sensor or interlock may sometimes be bypassed or artificially held in position so that the burner system may continue to be used until a replacement is obtained. Such bypassing of a sensor or interlock is extremely undesirable, because a dangerous condition may subsequently develop which the burner control system can no longer sense due to the bypassing of the inoperative device.